I think it could be classified as a mechanical part so "MP".
Tho i tend to use just "M" for mechanical parts. Its not really a big deal what you call these oddball components. Just stick to a given standard for the more common components so that you don't call transistors "Q" "T" "TR" "NPN" on the same schematic and cause the BOM to look like a mess.
There are other designator markings that are commonly used deviating from standards such as "U" and "IC" or calling voltage regulators just the same "U" ,calling zenner diodes with the same "D", calling ferites as "L", calling LEDs as "LED".. etc It really doesn't matter just make sure you are consistent across a whole schematic.